When I purchased a new Kubota L-35 thirteen years ago, that came std with the 18" bucket; I opted for the additional 12" that cost another $565! Some tractor Kubota junk yards maybe able to help? Do a search on this site.
I would also check with many tractor rental yards, which specialize in having Kubota tractors? I have heard of a few that sell the backhoes, separate from the tractor, for optium selling value, by selling them separate when replacing their fleet. Also, some who buy from rentals may not want the BH?
Depending on you shipping cost, I would check with tractorsmart.com for a new price? They sell for a lot less than your dealers, and sell Kubota parts only. They maybe able to guide you somewhere for used, too?